lightningcss, https://github.com/parcel-bundler/lightningcss.
> An extremely fast CSS parser, transformer, and minifier written in Rust. Use it with Parcel, as a standalone library or CLI, or via a plugin with any other tool.
I've tried it on my site (https://mnt.io) with the CLI binary: the minifier is indeed compressing more than postcss. Pretty impressive. It's also faster, as expected.